This invention relates to a process for the extraction of hydrocarbons from a hydrocarbon-bearing substrate, for example an oil shale, tar sand or a bituminous coal. It also relates to an apparatus to be used in such a process.
It is well known that hydrocarbons can be extracted from such hydrocarbon-bearing substrates by heating particles of the substrate at a temperature of at least 400.degree. C. in the substantial absence of oxygen, and recovering the liberated hydrocarbons. In the case of oil shale this process is usually referred to as retorting and, in the case of bituminous coal, is called pyrolysis.
In a number of different known processes the heating of the substrate particles is carried out by heat exchange with a heat-bearing medium. Such a heat-bearing medium may, for example, be a solid medium consisting of inert particles which are heated in a separate vessel and then circulated through the extraction vessel. Sand may be used for this purpose.
Certain of the known retorting processes make use of the fact that the spent substrate, i.e. the substrate after extraction of the hydrocarbons, may contain appreciable amounts of coke. It has therefore been proposed to generate the heat required for the retorting process by complete or partial combustion of this coke to produce a hot spent substrate. This hot spent substrate may be employed as heat-bearing medium for the extraction process.
Many such processes are based simply on the heating of the substrate in a vessel, which amounts essentially to one perfectly mixed stage. However, the solids residence time distribution in such a vessel is far from optimal and it is better if the solids pass through the vessel in a staged manner.
In one example of such a staged retorting process for oil shale hydrocarbon-bearing substrate and hot spent substrate are introduced into the upper portion of an elongated vertical vessel and are passed downwards through the vessel under substantially plug-flow conditions, while an inert stripping gas is passed upwardly through the solids in countercurrent flow, in order to remove the liberated hydrocarbons.
A disadvantage associated with the use of such a countercurrent retorting process arises from the fact that there is often appreciable contact in the retorting vessel between the liberated hydrocarbons and the hot substrate. This contact can give rise to cracking of the hydrocarbons and hence to loss of product due to coke formation.